Is there a way to pass variables into Jinja2 parents?
You just need to declare that variable before extending the template, so the extended template will have access to to the variable trying
{% set trying = True %} <----------- declare variable
{% extends "/templates/template.html" %}
{% set title = "Welcome" %}
{% block head %}
{% endblock head %}
{% block content %}
{% endblock content %}
Few years later but hoping it may help latecomers
I think that you are looking to highlight active menus in the base layout and you need something like this
{% extends 'base.html' %}
{% set active = "clients" %}
then use can use "active" inside base.html
The example on the Jinja2 Tips and Tricks page explains this perfectly, http://jinja.pocoo.org/docs/templates/#base-template. Essentially, if you have a base template
**base.html**
<html>
<head>
<title> MegaCorp -{% block title %}{% endblock %}</title>
</head>
<body>
<div id="content">{% block content %}{% endblock %}</div>
</body>
</html>
and a child template
**child.html**
{% extends "base.html" %}
{% block title %} Home page {% endblock %}
{% block content %}
... stuff here
{% endblock %}
whatever python function calls render_template("child.html") will return the html page
**Rendered Page**
<html>
<head>
<title> MegaCorp - Home page </title>
</head>
<body>
<div id="content">
stuff here...
</div>
</body>
</html>